(1) Field of the Invention
The present invention relates to a polybutylene terephthalate type flame-retardant resin composition. More particularly, the present invention relates to a resin composition having an excellent flame retardancy, balanced performances, and improved molding processability characteristics (flowability, molding cycle, mold-staining resistance and the like), in which a staining of metals or a formation of decomposition products and volatile substances is controlled when the composition is used for molding electrical parts.
(2) Description of the Related Art
A polybutylene terephthalate homopolymer, a copolymer comprising at least 60% by weight of butylene terephthalate units, and a thermoplastic polyester blend comprising at least 60% by weight of polybutylene terephthalate (collectively called "PBT" hereinafter) are crystalline thermoplastic resins having excellent mechanical properties, electrical properties, physical properties and chemical properties, and accordingly, these PBT resins are widely used as engineering plastics for automobiles, electric appliances, electronic appliances, and the like.
In fields where a flame retardancy is required, for example, for the production of electrical parts and the like, PBT resin compositions which are rendered flame-retardant by an incorporation of an organic halogen type flame retardant, optionally together with a flame retardant assistant, are mainly used.
Nevertheless, the conventional PBT resin compositions comprising a flame retardant and a flame retardant assistant bring the following problems.
(1) The physical properties of the molded article are lowered by the presence of the flame retardant or the assistant used in combination with the flame retardant.
(2) PBT as the base resin is decomposed and deteriorated by the flame retardant or the assistant or the mutual action thereof when the composition is prepared or molded or when the molded article is used in a high-temperature atmosphere.
(3) The mold is contaminated or corroded with the flame retardant or the assistant or a decomposition by-product thereof, and the dimensional precision of the molded article or the processing efficiency is lowered.
(4) Due to the presence of the flame retardant or the assistant or a decomposition by-product thereof, or to the decomposition product of PBT described in (2) above, specks are formed in the molded article, or parts near the molded article are contaminated or discolored, resulting in a poor appearance thereof.
(5) Because of a poor compatibility between the flame retardant and the resin, a bleeding of the flame retardant to the surface of the molded article occurs, which results in the occurrence of various problems.
(6) The flowability of the resin is impeded and molding cannot be performed smoothly.
(7) When the molded article is practically used, metals present contiguously or close to the molded article are corroded or contaminated by the flame retardant or the assistant or a decomposition product thereof, and the functions of parts are impeded. For example, when the resin composition is used for electrical parts having an electric contact composed of a metal (for example, a housing for a switch or relay and a substrate), the contact metal becomes stained after a long period of use, and the electric resistance values of the contacts are drastically lowered.
(8) Because of the mutual action of the flame retardant and assistant when used in combination, the molded article is deteriorated and discolored under ultraviolet rays and the appearance and strength characteristics are poor. For example, when the resin composition is used for electrical parts generating ultraviolet rays (for example, a housing or holder of a fluorescent lamp), a deterioration or discoloration occurs, or a formation or adhesion of a decomposition product as mentioned in (4) above occurs, resulting in a spoiling of the appearance thereof.
Solutions to these problems have been independently proposed, and some of the problems have been satisfactorily solved. Nevertheless, it is still difficult to obtain a flame-retardant PBT composition in which all of the foregoing problems are fully resolved, and there is a strong demand for such a composition.
In general, PBT is molded and processed more easily than polyethylene terephthalate. Currently, however, there is often a need to further reduce the manufacturing cost of a molded article, and in particular, to improve the productivity by shortening the molding cycle (realization of so-called high-cycle molding).